1. Field of the Invention
The present invention relates to two-component polyurethane coating compositions which are highly resistant to environmental influences and to mechanical stresses.
2. Discussion of the Related Art
Two-component polyurethane (2C-PU) coatings based on aliphatic and cycloaliphatic polyisocyanates are known for their high weathering stability and resistance to yellowing and to chemicals. Consequently, one of their use is in the field of automotive OEM finishing, as topcoats or clear-coats (W. Wieczorrek in: Stoye/Freitag, Lackharze Resins for Coatings, p. 215 ff., C. Hanser Verlag, 1996). They are particularly notable for high resistance to environmental influences such as acid rain or bird droppings, in relation to conventional topcoats (R. Ryntz, Paint & Coatings Industry, March 1997, p. 36). The resistance to environmental influences of 2C-PU coatings based on polyisocyanates of hexamethylene diisocyanate (HDI) has been significantly improved by using a mixture of polyisocyanates of HDI and of isophorone diisocyanate (IPDI) (WO 93/05090).
Relatively high proportions of IPDI polyisocyanate in the polyisocyanate mixture leads, owing to the increase in the glass transition temperature, to a further improvement in the resistance to environmental influences, but also impairs the resistance to mechanical stresses, such as, for example, the scratch resistance or wash-line resistance (Industrie Lackierbetrieb 61 (1993), 30).
Against the demand for continually increasing quality requirements imposed on the OEM finishing of automobiles, the aim is for a further improvement in the required properties.
Accordingly, an object of the present invention is to find a 2C-PU coating composition featuring improved resistance to environmental influences without impairment the mechanical resistance properties, especially the scratch resistance or wash-line resistance. This object has been achieved by the 2C-PU coating compositions of the invention.